Thunderstone Search Appliance Manual

A list of the URL path extensions that the walker will accept. The
default list is empty, and indicates that all extensions are allowed.
Include the "." in each listed extension. Case is always
ignored. URLs with no extension are always allowed.

E.g. to accept MS-Word documents, add .doc to the list. Note
that if the list is non-empty, any extensions not listed will not be
walked.